This invention relates to a video signal processing circuit of, for example, a color television receiver and in particular, to a circuit for correcting a picture quality.
In a video signal processing circuit of a color television receiver, a distributed capacitance between connection lines increases due to the number of amplifier stages and additional circuits, resulting in a loss in a broadband component of the video signal and a lowered resolution of the eventual image. Even in an ideal transmission system which is free from any picture-quality degeneration, the reproduced image has less sharpness due to its restricted transmission band.
In order to correct such a lowered image sharpness, a contour correction circuit has been used as an image correction circuit. An image contour becomes more clearly defined through the application of a preshoot and overshoot to a video signal. The contour correction circuit comprises a second derivative circuit for differentiating an input signal twice, and an adder for adding together the input signal and twice-differentiated output signal. In the contour correction circuit, the rise and fall times of an output video signal are made shorter in the edges of its waveform, noting that the "rise time" is a time taken for the signal level to rise from 10 % to 90 % and that the "fall time" is a time taken for the signal level to fall from 90 % to 10 %. Thus the frequency band emerges as being apparently broader.
In this contour correction circuit, the rise and fall times of a video signal can be reduced at the edges of its waveform. In this case a preshoot and overshoot are applied to the edges of the signal waveform and an excessive contour correction is achieved. As a result, a black-and-white edge mark is formed on the reproduced image, thus presenting a "distorted image" problem.